Liver disease affects millions of people worldwide. Hepatocyte transplantation is considered a potential treatment for liver diseases and a bridge for patients awaiting liver transplantation, but its application has been hampered by a limited supply of hepatocytes. The capacity of embryonic stem (ES) cells to form multiple differentiated cell types in culture provides a unique model system for the generation of transplantable lineages and tissues for cell replacement therapies. The long-term goal of this project is to define the mechanisms regulating the specification of ES cell-derived endoderm to a hepatic fate, and maturation to functional hepatocytes that are able to repopulate a diseased liver. We will test the hypothesis that endothelial cells support ES cell-derived endoderm cell survival, growth, hepatic specification and maturation during liver development and we will subsequently determine the mechanisms underlying these cell-cell interactions. Similar to what occurs during normal liver development, liver transplant repopulation requires highly orchestrated cell-cell interactions. We will identify the important molecular players in the crosstalk between endothelial cells and hepatic cells that could be relevant for promoting liver regeneration following transplantation with ES cell-derived hepatic cells. The characterization of these cell-cell interactions will provide a unique model for basic studies on the establishment of the hepatic lineage as well as provide functional hepatic cell for transplantation in pre-clinical models of liver cell replacement therapy.